When handling volatile liquids such as hydrocarbons including gasoline and kerosene, air-volatile liquid vapor mixtures are readily produced. The venting of such air-vapor mixtures directly into the atmosphere results in significant pollution of the environment. Accordingly, existing environmental regulations require the control of such emissions.
As a consequence, a number of processes and apparatus have been developed and utilized to recover volatile liquids from air-volatile liquid vapor mixtures. Generally, the recovered volatile vapors are liquified and recombined with the volatile liquid from which they were vaporized thereby making the recovery process more economical.
The initial vapor recovery systems utilized in the United States in the late 1920's and early 1930's incorporated a process combining compression and condensation. Such systems were originally only utilized on gasoline storage tanks. It wasn't until the 1950's that local air pollution regulations began to be adopted forcing the installation of vapor recovery systems at truck loading terminals. Shortly thereafter, the “clean air” legislation activity of the 1960's, which culminated in the Clean Air Act of 1968, further focused nationwide attention on the gasoline vapor recovery problem. As a result a lean oil/absorption system was developed. This system dominated the marketplace for a short time.
Subsequently, in the late 1960's and early 1970's cryogenic refrigeration systems began gaining market acceptance (note, for example, U.S. Pat. No. 3,266,262 to Moragne). While reliable, cryogenic systems suffer from a number of shortcomings including high horsepower requirements. Further, such systems require relatively rigorous and expensive maintenance to function properly. Mechanical refrigeration systems also have practical limits with respect to the amount of cold that may be delivered, accordingly, the efficiency and capacity of such systems is limited. In contrast, liquid nitrogen cooling systems provide more cooling than is required and are prohibitively expensive to operate for this type of application.
As a result of these shortcomings, alternative technology was sought and adsorption/absorption vapor recovery systems were more recently developed. Such a system is disclosed in a number of U.S. Patents including, for example, U.S. Pat. No. 5,871,568 to Gibson, the disclosure of which is fully incorporated herein by reference. Such systems utilize beds of solid adsorbent selected, for example, from silica gel, certain forms of porous mineral such as alumina and magnesia, and most preferably activated charcoal. These adsorbents have an affinity for volatile hydrocarbon liquids. Thus, as the air-hydrocarbon vapor mixture is passed through the bed, a major portion of the hydrocarbons contained in the mixture are adsorbed on the bed. The resulting residue gas stream comprising substantially hydrocarbon-free air is well within regulated allowable emission levels and is exhausted into the environment.
It should be appreciated that the beds of adsorbent used in these systems are only capable of adsorbing a certain amount of hydrocarbons before reaching capacity and becoming ineffective. Accordingly, the beds must be periodically regenerated to restore the carbon to a level where it will effectively adsorb hydrocarbons again. As a result vapour recovery systems of the type disclosed in the Gibson patent generally include at least two separate reaction vessels holding two separate beds of adsorbent. This allows one bed to be used to recover vapor while the other bed is regenerated. While such a system is effective, it is also relatively expensive to build and maintain for proper operation as it requires two reaction vessels, two beds of adsorbent and relatively complicated piping, valving and control systems.